In recent years, as the number of cellular phone users increases, it has become difficult to secure communication lines for cellular phones of a single cellular phone system. Thus, cellular phones each of which responds to more than one cellular phone communication system are demanded on the market. Hence, a switching circuit for switching the frequencies of a number of communication systems without allowing the communication systems to affect other parts is demanded for an antenna.
In order to meet the needs of the market, in a conventional semiconductor apparatus where a switching circuit is integrated by semiconductors (for example, Japanese Patent Laid-Open No. 9-153781), shunt FETs are provided respectively for through FETs (field-effect transistor) allowing the passage of a signal and through FETs for interrupting a signal in signal paths (through). Before a desired inputted high frequency signal leaks to other parts through the interrupting through FET, the high frequency signal is released to a part not affecting the parts of other frequencies or cellular phone systems through the shunt FET allowing the passage of a signal, so that a high isolation is obtained between changeover terminals.
The conventional semiconductor apparatus with an integrated switching circuit will be discussed below in accordance with the accompanying drawings. The drawings only illustrate control signal circuits for switching signal paths by through FETs and capacitor circuits for grounding output terminals to GND in a high frequency manner. DC power supply circuits for supplying power to the FETs are omitted.
First, the following will discuss a “Single Pole Three Throw (hereinafter, referred to as SP3T) switch” having a switching function of switching three output paths for one input path.
FIG. 5 is a circuit diagram showing the configuration of a semiconductor apparatus according to conventional example 1 and is also a circuit diagram showing an SP3T switch. A method of using the SP3T switching circuit will be discussed below. In FIG. 5, reference numeral RF1 denotes an input terminal of a high frequency signal, reference numerals RF2, RF3, and RF4 denote output terminals of a high frequency signal, reference numerals 5, 6, and 7 denote gate bias terminals, reference numerals 5-a, 6-b, and 7-c denote feedthrough FETs (abbreviated as through FETs), and reference numerals 5-a_R, 6-b_R, and 7-c_R denote gate resistors. In FIG. 5, the through FETs 5-a, 6-b, and 7-c are N-type channel FETs.
Referring to FIG. 6, an operating principle of transmitting a high frequency signal (RF) from the input terminal RF1 to the output terminal RF4 will be discussed below based on a method of switching through FETs in the semiconductor apparatus of FIG. 5.
In order to turn on the FET, 0 V or a positive voltage not higher than a Schottky barrier voltage (about 0.7 V) is applied as a voltage between the gate and drain terminals and the gate and source terminals of the FET. Thus, the FET is forward biased and the channel of the FET is opened, so that an RF signal is transmitted between the input terminal RF1 and the output terminal RF4. In order to turn off the FET, a voltage between the gate and drain of the FET and a voltage between the gate and source of the FET are set to the equivalent voltage of the threshold of the FET or lower. Hence, the channel of the FET is closed and the FET is turned off.
FIG. 6 is a circuit diagram showing an operation when the semiconductor apparatus of conventional example 1 is turned on between the input terminal RF1 and the output terminal RF4. FIG. 6 shows that the gate bias terminals 5 and 6 are set to L (off) and the gate bias terminal 7 is set to H (on) in response to a control signal fed with voltage from an external power supply. Under this condition, a voltage drops on the resistor 7-c_R, so that the through FET 7-c is turned on. Hence, when the through FET 7-c is turned on, passage is enabled through a signal path between the input terminal RF1 and the output terminal RF4. Example applications for the operating principle of the SP3T switch include a Single Pole four Throw (SP4T) switch and a Single Pole five Throw (SP5T) switch.
The items of characteristic evaluation of an antenna switch using FETs include an insertion loss, an isolation, harmonics, a ratio of leakage power between adjacent channels. An insertion loss is a degree of attenuation of an RF signal passing between the source and drain of the through FET 7-c when the RF signal is transmitted from the input terminal RF1 to the output terminal RF4. An isolation is a degree of leakage of an RF signal to the other output terminals RF2 and RF3 when the RF signal is transmitted from the input terminal RF1 to the output terminal RF4. A low insertion loss and a high isolation are demanded as ideal characteristics of a switching circuit used as an antenna switch. An insertion loss and an isolation are also items of characteristic evaluation of a small signal.
On the other hand, in the switching circuit shown in FIGS. 5 and 6, only the through FETs are present on the signal paths. Thus, an insertion loss increases at a specific frequency of a high frequency signal transmitted to each of the signal paths and an isolation deteriorates between the on path and the off path of the through FET.
In order to prevent these problems, the shunt FET is arranged between the output terminal and the through FET on the signal path. A unit for particularly reducing an insertion loss and improving an isolation will be discussed below in accordance with an illustration of SP3T.
FIG. 7 is a circuit diagram showing the configuration of a semiconductor apparatus of conventional example 2 and is also a circuit diagram showing SP3T type switches constituting shunt FETs near output terminals RF2, RF3, and RF4. The shunt FETs are configured near the output terminals, respectively. Referring to FIG. 8, the following will particularly describe a switching principle allowing passage between an input terminal RF1 and the output terminal RF4 when the SP3T type switching circuit of FIG. 7 is operated.
A through FET 7-c on ON path between the input terminal RF1 and the output terminal RF4 is turned on, and a shunt FET 5-c between the output terminal RF4 and a capacitor C10 having one terminal connected to GND 10 is turned off. On OFF path between the input terminal RF1 and the output terminal RF2 and between the input terminal RF1 and the output terminal RF3, a through FET 5-a and a through FET 6-b on OFF path are turned off, and a shunt FET 6-a between the output terminal RF2 and a capacitor C8 having one terminal connected to GND 8 and a shunt FET 7-b between the output terminal RF3 and a capacitor C9 having one terminal connected to GND 9 are turned on. Consequently, an RF signal is transmitted from the input terminal RF1 to the output terminal RF4 and an RF signal transmitted to the OFF path is grounded to the GND 8 and GND 9 through the capacitor C8 and the capacitor C9, thereby reducing the leakage of the RF signal to the output terminal of the OFF path.
However, as shown in FIGS. 7 and 8, the semiconductor apparatus where the conventional switching circuit is formed requires six voltage control terminals for controlling on/off of the FETs on the three paths and the shunt FETs connected to the FETs, for example, in the SP3T, resulting in a complicated circuit configuration for controlling the switching operation of the semiconductor apparatus having such a switching function.
In other words, when the switching circuit principle illustrated in FIG. 8 is used based on the configuration of FIG. 7, the terminals for controlling the FETs have to operate separately, and thus control terminals 5′, 6′ and 7′ for the shunt FETs are necessary in addition to the control terminals 5, 6, and 7 for the feedthrough FETs. Thus, six control terminals are necessary in total. Alternatively, Logic for controlling terminals in a complicated configuration has to be used to reduce the number of times of control on the control terminals.
The present invention is devised to solve the problem and provides a semiconductor apparatus which makes it possible to reduce the number of control terminals required for switching the through paths of a high frequency signal, simplify the circuit configuration for controlling the terminals, improve an isolation characteristic between the on path and off path of the through FET, and obtain a sufficiently high isolation.